1. Field of the Invention
The present invention relates to a reflection-type or transflective color liquid crystal display device containing a color filter therein, capable of a multicolor display.
2. Description of the Related Art
Conventionally, a reflection-type liquid crystal display device for performing a monochrome display using a TN (twisted nematic) liquid crystal cell or an STN (super twisted nematic) liquid crystal cell, has been mainly used as a display device of a portable electronic device such as an electronic notepad, an electronic dictionary, a Personal Digital Assistant (PDA), or the like.
However, in recent years, due to the increases in amount and diversification of information, there has been a growing demand for a color display and thus reflection-type or transflective color liquid crystal display devices containing color filters therein have been vigorously developed.
The reflection-type color liquid crystal display devices containing color filters therein are broadly classified into the following three types.
The first type is a reflection-type color liquid crystal display device without a polarizing film. This type includes one using a Guest-Host liquid crystal in which a dichroic dye or a black dye is mixed in a liquid crystal material which is sealed in a liquid crystal cell, one using a polymer-dispersion liquid crystal in which a liquid crystal material is dispersed in a polymer, or so on.
The reflection-type color liquid crystal display device using a Guest-Host liquid crystal is disclosed in, for example, Japan Patent Laid-open No. S59-198489 (JP, 59-198489, A). The reflection-type color liquid crystal display device using a polymer-dispersion liquid crystal is disclosed in, for example, Japan Patent Laid-open No. H5-241143 (JP, 5-241143, A).
Since none of them uses a polarizing film, they have excellent brightness but low in contrast, and thus they have not been realized for practical use yet.
The second type is a reflection-type color liquid crystal display device using one polarizing film and containing a reflector inside the liquid crystal display device. This type is further divided into two types, and one type uses a self-contained reflector with a reflection face of a mirror surface which has a light diffusing layer on the reflection face side, and the other type uses a self-contained reflector with light scattering properties in the reflection face.
Both types are good in chroma with little color mixture because light is reflected inside the liquid crystal cell. Further, they also have excellent brightness because of only one polarizing film, though there is a problem of low contrast.
As for the type using a self-contained reflector with a reflection face of a mirror surface, it is bright in a direction of regular reflection of incident light, but it becomes abruptly darker at other angles, and thus there is also a problem that the viewing angle characteristic thereof is quite poor.
Also the other type using a self-contained reflector with light scattering properties in the reflection face has a problem that it is difficult to control the light scattering properties and the fabricating process becomes complicated.
The reflection-type liquid crystal display device using one polarizing film as described above is disclosed in, for example, Japan Patent Laid-open No. H3-223715 (JP, 3-223715, A).
The third type is a color liquid crystal display device made by providing a color filter in a liquid crystal cell of a typical monochrome liquid crystal display device using two polarizing films and providing a reflector outside the liquid crystal cell. Since this color liquid crystal display device uses two polarizing films, it is excellent in contrast, but it has the disadvantage of a dark display.
However, a reflection-type polarizing film is used in place of the polarizing film and the reflector which are provided on the opposite side to the visible side of the liquid crystal cell, thereby improving brightness of the display, and this color liquid crystal display device is considered adequate for practical use.
The color liquid crystal display device using a reflection-type polarizing film as described above is disclosed in, for example, Japan Patent Laid-open No. H10-3078 (JP, 10-3078, A).
The third type of color liquid crystal display device, however, has spectral characteristics that its reflectance is low on the long wavelength side when performing a reflection-type color display.
Therefore, the chroma of red is low and the background color has green mist, and thus a color display goes out of color balance, bringing about a problem of bad visibility.
An object of the present invention is to improve the spectral reflectance characteristics in a color liquid crystal display device of the third type, and especially to improve the chroma of red by increasing the reflectance on the long wavelength side, thereby enabling a multicolor display with high chroma on a white background.
The color liquid crystal display device according to the present invention is configured as follows in order to attain the above-described object.
An STN liquid crystal cell comprises a first substrate having first electrodes, a second substrate having second electrodes, a color filter provided on any one of the first substrate and second substrate, and a liquid crystal layer composed of a super twisted nematic liquid crystal sealed between the first substrate and second substrate and aligned such that major axes of liquid crystal molecules are twisted at an angle of 180xc2x0 to 270xc2x0.
A polarizing film is provided on the outside of the second substrate which is the visible side of the STN liquid crystal cell, and a twisted retardation film is provided between the polarizing film and the second substrate. Further, a diffusing layer, a reflection-type polarizing film, and a light absorbing layer are provided outside the first substrate in order.
Further, the super twisted nematic liquid crystal of the liquid crystal layer is aligned such that major axis directions of liquid crystal molecules located at the middle between the first substrate and second substrate are parallel to a horizontal axis of a display screen.
When the light absorbing layer in the reflection-type color liquid crystal display device is a translucent-type light absorbing layer and a backlight is disposed outside the translucent-type light absorbing layer, a transflective color liquid crystal display device can be made.
In this case, it is preferable that an average transmittance of the translucent-type light absorbing layer is in a range of 20% to 60%.
Further, it is desirable that a difference in absolute value between a twist angle of the liquid crystal layer and a twist angle of the twisted retardation film is 10xc2x0 to 30xc2x0.
It is suitable that red filters, green filters and blue filters are formed alternately in stripes shape as the color filter.
The above and other objects, features and advantages of the invention will be apparent from the following detailed description which is to be read in conjunction with the accompanying drawings.